Photoconductive compositions comprising zinc oxide and methods for using such



United States Patent 3 376,134 PHGTOCGNDUCTIVE COMPOSITIQNS COMPRIS- INGZHNC OXIDE AND METHODS FOR USING SUCH Frederick A. Stahly and WestlakeL. Goehring, Rochester,

N.Y., assignors to Eastman Kodak Company, Rochester, N.Y., a corporationof New Jersey No Drawing. Filed Mar. 16, 1965, Ser. No. 440,315

9 Claims. (Cl. 96--1.8)

This invention relates to improved coating compositions for use asphotoconductive layers in electrophotographic elements and to elementscomprising such coatlllgS.

It is known that photoconductive zinc oxide can be employed inphotoconductive insulating layers coated on paper, to make elementsuseful in electrophotography.

In a known electrophotography process employing zinc oxide inphotoconductive layers, the photoconductive insulating coating is firstgiven a blanket negative electrostatic charge, in the substantialabsence of actinic radiation, by means of a corona discharge. Thephotoconductive layer is then exposed imagewise to actinic radiation.Exposed areas of the photoconductive coating will lose, Wholly or inpart (depending upon amount of exposure), the negative electrostaticcharge while unexposed portions of thephotoconductive layer retain thenegative electrostatic charge. Photographic speed is proportional to theexposure required to dissipate the charge and is measured by methodssimilar to those used for measuring photographic speed of silver halidefilms. The resulting electrostatic image can be developed byconventional methods, for example using a pigmented resin powder whichhas a charge opposite to the negative charge of the unexposed areas ofthe photoconductive layer. The pigmented powder is attracted to thenegatively charged areas and can then be affixed to the photoconductivelayer by simply heating the resinous vehicle of the toner to fuse it tothe photoconductive coating. Various other means of developing thelatent photoconductive image to a visible image have been described inthe prior art.

A disadvantage of zinc oxides normally used in such photoconductivelayers is that the photosensitivity of the zinc oxide layer normally isat its greatest in the ultraviolet region of the spectrum, whereas anexposing source may have its maximum output in a visible region of thespectrum. Various sensitizing materials are known to increase thesensitivity of zinc oxide to the visible region of the spectrum andthese may be incorporated in coating compositions of the presentinvention. By addition of sensitizer dyes, the photographic speed ofphotoconductive layers containing zinc oxide can be increased by afactor of 200 or more over that of the unsensitized composition.However, at speeds greater than about six times that of the unsensitizedcomposition, the sensitizers begin to cause objectionable coloration.

Therefore, an object of the present invention is to provide improvedphotoconductive coating compositions com prising zinc oxide for makingelectrophotographic elements having improved photographic speed. It isanother object of the present invention to provide photoconductivecompositions that can employ lower concentrations of sensitizing dyes toachieve a given photographic speed. It is still another object toprovide electrophotographic elements based on photoconductive zinc oxideand having improved photographic speeds and less coloration. Otherobjects will be apparent from the following detailed description andclaims.

In accordance with the present invention, to a conventionalphotoconductive zinc oxide coating composition which contains a suitableinsulating binder, and preferably containing at least in part a siliconeresin, we add the product of dimerization of a C to C unsaturated fattyacid. The dimerization product, which is preferably formed from anunsaturated fatty acid having from 16 to 22 carbon atoms, is generallyemployed in a concentration of at least 3 percent based on the weight ofthe zinc oxide. As indicated hereinabove, the photoconductivecomposition may contain in addition to the said dimerization product aphotosensitive dye such as a merocyanine or carbocyanine dye or any ofthe other photosensitive dyes that are suitable for sensitizingphotoconductive zinc oxide.

The coating compositions of the present invention may be mixed inaccordance with techniques generally known in the art. Incorporation ofthe fatty acid dimerization product in the coating requires no specialprocessing techniques. For example, compositions of the presentinvention can be prepared by blending the ingredients with a liquiddiluent in conventional mixing equipment. Hydrocarbon solvents and polarorganic solvents such as alcohols, ketones, and esters are usuallyemployed as liquid diluents. In general, it is preferred to addphotoconductive zinc oxide, usually French process zinc oxide, to theliquid diluent first and to follow by addition of a silicone resin (whenemployed), the sensitizer dye, the fatty acid dimerization product andother additives, and last, add the other dielectric binder resin.However, it is to be pointed out that the order of addition ofingredients to make the composition is not a critical element of thepresent invention.

The resulting composition is then thoroughly mixed and coated onto asuitable support, usually paper and the diluent is evaporated. It ispreferred to evaporate substantially all of the solvent in order toavoid interference with the dielectric properties of the coating.Coating techniques have been developed and are known in the art forapplication of photoconductive zinc oxide coating compositions tosupports and these conventional coating methods are suitable for coatingcompositions of the present invention.

The resins employed as binders may be selected from a wide variety ofresinous materials. While there are, of course, some preferred binderresins, in general, we may use any resin that can be dried at a safetemperature, that has good dielectric properties and that adheres wellto the zinc oxide and the support. In order to improve electricalresponse at high humidity, it is preferred to employ a silicone resinwhich usually is used in combination with other resins, but which mayalso be used by itself. In preferred coatings in accordance with thepresent invention, silicone resins are employed in a concentration rangeof about 5 to 50 percent of the binder composition. Other preferredresins suitable for use in binder compositions include polystyrene andstyrene copolymer resins, ABS resins, polyvinyl halide resins, polyvinylacetate resins, acrylate resins, alkyd resins, and silicone resins.Resins suitable for use as binders in zinc oxide electrophotographicelements have been described in the prior art and need not be detailedhere.

The dimerization product of an unsaturated fatty acid monomer, employedin the compositions of the present invention, is prepared by a processfor polymerization of the monomer at the double bond. Such products areavailable commercially. The dhnerized acid isthe principal ingredient ofthe dimerization product, which usually also contains lesser amounts ofunreacted fatty acid monomer and some higher polymerization products,i.e., trimers, tetramers, etc. For convenience, in this specification,we may refer to the dimerization product by the term dimer. Theinvention includes use of dimers of such unsaturated fatty acid monomersas oleic acid, palmitoleic acid, petroselenic acid, and erucic acid inzinc oxide photoconductive coating compositions. The preferredunsaturated acid monomers are those containing from 16 to 22 carbonatoms and especially preferred are the unsaturated fatty acidscontaining 18 carbon atoms. The acid monomer may contain one double bondor may contain, more than one double bond as in the case of linoleicacid or linolenic acid. The dimers are employed in a concentration of atleast 3 percent based on the weight of the zinc oxide. Suitableconcentrations range from about 3 to about 1 percent by weight of thezinc oxide. Still higher concentrations are operable, but beyond about10 percent the additional dimer does not effect much additionalimprovement.

The unsaturated fatty acid dimers improve the speed of Zinc oxidecontaining photoconductive compositions about equally for whatever wavelength of light is employed; hence, photoconductive compositions of zincoxide and the dimer retain maximum sensitivity in the ultravioletregion. If it is desired to shift or extend maximum sensitivity to thevisible region, it is still necessary to add sensitizers, usually one ormore of the known sensitizing dyes. The presence of such sensitizers inmost cases Will not interfere with the effect of the dimers and in fact,presence of the dimer will reduce the amount of dye necessary to obtaindesired speed at the sensitized Wave lengths.

The preferred zinc oxide photoconductive materials are of relativelysmall particle size, generally less than 0.5 micron mean diameter. Suchzinc oxide materials are readily available and can be purchased under avariety of trade names, such as XX78 and Photox 801 (New Jersey ZincCompany), etc. Sufficient binder should be employed to insulate each ofthe zinc oxide particles from the surrounding particles in thephotoconductive insulating coatmg.

Exposure of a charged photoconductive insulating layer to actinicradiation causes a loss or reduction of electrical resistivity inexposed areas of the photoconductive coating and a consequent dischargeof the static charge in such areas. The degree of discharge will dependon the intensity and time of exposure. The resulting latentelectrostatic image can then be developed to a visible image by any ofseveral methods known in the art. A useful method of developing thelatent electrophotographic image is the magnetic brush process describedin US. Patent No. 2,874,063 patented Feb. 17, 1959 to Harold G. Grieg.

Uses and advantages of certain preferred embodiments of the inventionare illustrated by the following example.

EXAMPLE This example describes preparation of a number ofphotoconductive compositions with and without a preferred additive ofthe present invention and compares the photographic speeds obtained withcoatings thereof.

Composition A To a Waring Blendor were added 314.7 g. of toluene, 256.0g. of zinc oxide (modified with 1.0 10 eq. HCl per 56 g. of zinc oxide),21.3 g. of a 60 percent solution of a silicone resin (Dow-CorningZ-6018) in toluene, and 170.6 g. of a 30 percent solution of a speciallyheat treated 50/50 n-butyl-isobutyl methacrylate copolymer (Du Pont Co.Lucite 2046) in toluene in that order and sheared for 20 minutes. Then37.4 g. of methyl alcohol was added to the mixture which was gentlystirred for 5 minutes.

Composition B To a Waring Blendor were added 314.3 g. of toluene, 256.0g. of zinc oxide (modified with 1.0 eq. HCl per 56 g. of Zinc oxide),20.8 g. of a 60 percent solution of a silicone resin (Dow-CorningZ-6018) in toluene, and 171.5 g. of a 30 percent solution of a speciallyheat treated 50/50 n-butyl-isobutyl methacrylate copolymer (Du Pont Co.Lucite 2046) in toluene in that order 4 and sheared for 20 minutes. Then0.00690 g. of a sensitizer dye having the following composition:

Percent.

Crystal violet 28.6

Fluorescein 34.6 3-carboxymethyl-5,3-eth.yl-2-( 3-hydro)benzothiazolylidene rhodanine 36.7

dissolved in 37.4 g. of methyl alcohol was added to the mixture whichwas gently stirred for 5 minutes.

Composition C A mixture of 1905 g. of toluene, 95.2 g. of a 60 percentsolution of the said silicone resin at 1143 g. of the said zinc oxidewas processed through a laboratory model Sand Grinder. The mixture wasthen sheared for 15 minutes in a Polytron mixer after the addition of34.3 g. of the product of dimerization of unsaturated C fatty acid(Emery 3020-R dimer acid, Cincinnati, Ohio). This compositioncontainsabout 5 percent of the C acid, 73 percent of the. C dicarboxylicdimer acid and 22 percent of the C tricarboxylic trimer acid. Themixture was sheared for another 30 minutes following the addition of761.8 g. of the 30 percent methacrylate in toluene solution describedfor Composition A and 1153 g. of additional toluene.

To 658.2 g. of this composition was added 27.8 g. of

methanol and the mixture was stirred for 15 minutes.

Composition D To 658.2 g. of Composition C without the added methanolwas added 0.00513 g. of the sensitizer dye of Composition B dissolved in27.8 g. of methanol and the mixture was gently stirred for 15 minutes.

Composition E To 658.2 g. of Composition C Without the added methanol,was added 0.0102 6 g. of the sensitizer dye of Composition B dissolvedin 27.8 g. of methanol and the mixture was gently stirred for 15minutes.

Compositions A through B were coated on St. Lawrence C2S paper at a dryWeight coverage of 3.0 g./ft. The

so-coated papers were .then charged under a corona discharge and exposedfor one-half second in a sensitometer to tungsten illumination. Theexposed coating was then developed by magnetic brush development usingsmall iron carrier particles and black pigmented sulfur toner- TABLEComposition: Relative white light speed A 10 B 25 C 20 D 75 E It will beapparent that Composition A must be compared to Composition C andComposition B to Com-= positions D and E. Despite the higherconcentration of sensitizer dye in Composition E, n0 objectionable stainwas observed.

The foregoing example is intended to illustrate the preparation ofcertain preferred compositions of the present invention and advantagesto be derived therefrom and a preferred use for the coatingcompositions. The example is not intended to limit the scope of theinvention to the embodiments described. Other resins and sensitizer dyeshaving the above-indicated requisite properties may be employed. Otherdimers derived from dimerization C to C fatty acids may be similarlyemployed. Variations and modifications will be apparent to personsskilled in the art and are included within the scope of this inventionas defined in the appended claims.

What is claimed is:

1. A photoconductive coating composition comprising a high dielectricresinous binder insulator having uniforml dispersed thereinphotoconductive zinc oxide particles and at least 3 percent 'by weightof the zinc oxide of the product of dimerization of an unsaturated fattyacid monomer in which said fatty acid monomer has from 12 to 24 carbonatoms.

2. A photoconductive composition comprising a high dielectric resinousbinder insulator having uniformly dispersed :therein photoconduotivezinc oxide, a sensitizing dye, and at least 3 percent by weght of theZinc oxide of the product of dimerization of an unsaturated fatty acidmonomer in which the fatty acid monomer has from 12 to 24 carbon atoms.

3. The photoconduc-tive composition of claim 1, wherein the highdielectric resinous binder insulator contains from 5 to 50 percent ofthe mixture of a silicone resin.

4. The 'photoconductive composition of claim 1, Wherein the highdielectric resinous binder insulator is a mixture of a methacrylateresin and from 5 :to 50 percent 'by weight of the mixture of a siliconeresin.

5. The composition of claim 1, wherein the dimerization product isderived from an unsaturated fatty acid monomer having 18 carbon atoms.

6. The photoconductive composition comprising a mix- .ture of a.methaicrylate resin and from 5 to 50 percent by weight of the mixtureof a silicone resin having uniformly dispersed there-in photoconductivezinc oxide, a sensitizing dye, and from 3 to 10 percent based on theWeight of the zinc oxide of a product of dimerization of an unsaturatedfatty acid monomer in which the acid monomer has 18 carbon atoms.

7. The photoconductive composition of claim 6, wherein the methacrylateresin is an n-butyl-isobutyl methacrylate copolymer.

8. A photographic element for electrophotography comprising a .papersupport and a relatively thin layer of a composition comprising a highdielectric resinous binder insulator having unifiormly dispersed thereinphotoconductive zinc oxide particles and at least 3 percent by weight ofthe zinc oxide of the product of dimerization of an unsaturated fattyacid :monomer in which the fatty acid has from 12 to 24 carbon atoms.

9. The process of claim 8, wherein the resinous binder is a mixtureconsisting of a meth acrylate copolymer and from 5 to percent by weightof the mixture of a silicone resin and wherein the dimerization productis a dimer of an unsaturated fatty acid monomer containing 18 carbonatoms.

References Cited UNITED STATES PATENTS 5/1964 Stahley 96-1 4/1966Sh-ulm'an 961

1. A PHOTOCONDUCTIVE COATING COMPOSITION COMPRISING A HIGH DIELECTRICRESINOUS BINDER INSULATOR HAVING UNIFORMLY DISPERSED THEREINPHOTOCONDUCTIVE ZINC OXIDE PARTICLES AND AT LEAST 3 PERCENT BY WEIGHT OFTHE ZINC OXIDE OF THE PRODUCT OF DIMERIZATION OF AN NSATRATED FATTY ACIDMONOMER IN WHICH SAID FATTY ACID MONOMER HAS FROM 12 TO 24 CARBON ATOMS.